The present invention relates to a control rod driving system and more particularly to a control rod driving hydraulic system of the control rod driving system for supplying a driving water to a control rod driving mechanism.
A prior art control rod driving system of a general boiling water reactor plant will be described with reference to FIGS. 7 and 8. FIG. 7 represents the construction of a control rod driving mechanism 1 and a water pressure, i.e. hydraulic, control unit 2, and FIG. 8 represents an outline of a control rod driving hydraulic system.
The control rod driving mechanism 1 employs a hydraulic piston driving system and principally comprises a driving piston 1a, an index tube 1b and a locking mechanism 1c. The driving piston 1a is mounted on a lower portion of the index tube 1b, provided with vertical pressure receiving faces and operates to insert and extract a control rod in accordance with a difference in pressure between the upper and lower surface portions. Further, the index tube 1b is provided with a locking groove on its outer peripheral surface, locked after it's moved by a constant stroke by the locking mechanism 1c such as a ratchet type collet finger or the like, thus holding the control rod fixedly at a predetermined position.
The control rod driving hydraulic system supplies hydraulic pressure and flow necessary for operation of the control rod to adjustment. The system comprises the water pressure control unit 2 attached to each control rod 1 and a control rod driving water pressure supply system common to all control rods.
The control rod driving hydraulic system is that for supplying a demineralized condensate to the control rod driving mechanism 1 from a condensate demineralizer 3 during operation of the plant, and as shown in FIG. 8, the system comprises principally a driving water pump 4, a suction filter 5, a driving water filter 6, a flow nozzle 7 working as a flowmeter, flow control valves 8 and 9, pressure control valves 10 and 11, stabilizing valves 12 and others. The driving water fed into the driving water pump 4 from the condensate demineralizer 3 flows partly into a charing water header 13 as a scrum water pressure unit by way of two kinds of filters, namely the suction filter 5 and the driving water filter 6, for removing foreign materials which are capable of causing an extra ordinary operation of the control rod driving mechanism 1 and others. Further, it flows partly into a driving water header 14 or a driving hydraulic system for actuating the control rod driving mechanism 1 and partly into a cooling water header 15 of a cooling water pressure system of a control rod driving mechanism piston seal by way of the flow control valves 8 and 9 for retaining a charging water pressure.
The stabilizing valves 12 are constructed of two electromagnetic, i.e. solenoid, valves provided in parallel. A quantity of flow passing one stabilizing valve 12 is normally equal to the quantity of flow necessary for the control rod driving mechanism 1 to insert the control rod, and a quantity of flow passing the other stabilizing valve 12 is adjusted to be equal to the quantity of flow necessary for withdrawal. Water coming out of the stabilizing valves 12 is circulated to the cooling water piping. That is, when the one stabilizing valve 12 is closed at the time of driving operation, a predetermined quantity of flow runs toward the control rod driving mechanism 1, thus operating to keep a hydraulic pressure control valve 10 constant.
Further, a piping from a drain header 16 is connected to the stabilizing valve 12 on the downstream side, water flowing from the driving water header 14 to the drain header 16 by way of the control rod driving mechanism 1 at the time of driving is fed to the cooling water header 15, and then released into a reactor pressure vessel through the other water pressure control unit 2.
At the time of normal operation, only a cooling water flows, a steady flow sufficient enough to cover cooling of all the control rod driving mechanisms 1 runs, and the pipings same in the number of control rods communicate further with each water pressure control unit 2 from each of the headers 13 to 16.
The control rod driving hydraulic system is equipped with a condensate storage tank 18, thus a source of the driving water can be transferred to the condensate storage tank 18 from the condensate demineralizer 3.
The water pressure control unit 2 supplies charging water, cooling water and driving water from the control rod driving hydraulic supply system to the control rod driving system, and is provided in one unit per one control rod.
The water pressure control unit 2 is that of having unitized, as shown in FIG. 7, four directional control valves 21, 22, 23 and 24 and two scrum valves 25 and 26 en bloc.
The water pressure control unit 2 operates as follows.
When inserting the control rod, the insert directional valves 21 and 22 are opened, a hydraulic pressure exerted on the driving water header 14 is provided to a lower surface of the driving piston 1a, and thus water on the upper surface is released to the drain header 16. When extracting, on the contrary, the insert directional control valves 21 and 22 are opened in a short time, the index tube 1b is lifted slighly to an easy unlocking of the collet finger, then the withdraw directional control valves 23 and 24 for withdrawal are opened to apply a driving water pressure to an upper surface of the driving piston 1a, and water on the lower surface is released to the drain header 16. The collet finger is spread out along a guide to separate from the index tube 1b, and is thus ready for extraction of the control rod. Then, by opening the scrum valves 25 and 26 at the time of scrum operation, a high pressure water is poured into the lower surface of the driving piston 1a from the charging water heater 13, water on the upper surface of the driving piston 1a is released to a scrum discharging header 27 of atmospheric pressure, thereby realizing a quick scrum operation.
In the construction described above, by changing a position of the control rod in a reactor core according to a manual control signal, the control rod driving system operates for adjustment of outputs at the time of low output, adjustment of a long term reactivity and control of a core output distribution.
The condensate purified by the filters of the control rod driving water is utilized as purge water for a mechanical seal of a reactor coolant recirculation pump. The mechanical seal is exchanged with another one as an expendable item at every periodical inspection time, but the mechanical seal in use contacts operation water in the reactor, so that the mechanical seal may be radioactive. For this reason, the condensate of a pressure higher than the reactor pressure is used as the purge water to prevent exposure to workers.
In a prior art control rod driving system, two kinds of filters are intended for purifying a driving water of the control rod driving mechanism 1 for driving a control rod and removing foreign materials such as cladding and the like. Namely the suction filter 5 of about 25 .mu.m or so in absolute performance of filtration and the driving water filter 6 of about 50 to 70 .mu.m or so in absolute performance of filtration are provided on an inlet side of the driving water pump 4 for pressurizing the driving water and on an outlet side thereof respectively, thereby filtering the driving water of the control rod driving mechanism 1.
The purified condensate is fed as the purge water for the mechanical seal of the reactor coolant.
However, such performances of filtration are still not satisfactory in removing foreign materials such as cladding and the like thoroughly, and thus are capable of causing the foreign materials to come into the control rod driving mechanism 1 and the water pressure control unit 2. There may cause a case where the biting of the foreign materials to the mechanical seal of the reactor coolant recirculation pump may cause the leakage of the reactor water. Further, there may cause a case where the air comes into the control rod driving hydraulic system at the time of operation for disassembly and check of each equipment and change of a water source, and since the air having come thereinto exerts an influence on a driving performance of the control rod driving mechanism 1, a work for withdrawing the air must be done carefully on each occasion.